This invention relates to the immobilization of modular circuit components to reduce potentially deleterious effects of sudden movements, such as shock and vibration, under handling and when in use in a circuit. In particular, this invention is directed to a case or box for enclosing a modular circuit component in comparatively immobile and secure position without the need for surrounding the component with potting material and to the component so enclosed.
Modular circuit components performing various functions have found increasing application in areas, such as aerospace and military uses, which require that they withstand high impact, shock and vibration and similar potentially harmful operating characteristics. To aid in meeting high shock, impact and vibration specifications, modular circuit components have been encapsulated with potting material within an enclosure. This material is either of the hard pot or soft pot type, the difference being that a component potted with soft pot can be depotted in the event that its electrical characteristics are not satisfactory and correction is required. If a component is potted with hard pot it is difficult to remove the potting material without at the same time destroying or damaging the component.
A method of soft potting involves placing a soft rubber-like material, such as that available under the tradename Silgard, in the shell of a case to partially fill the case. The circuit component is then placed in the case and maintained spaced from the case bottom. A cover with an opening is placed over the top of the case shell and the edges sealed with an epoxy or similar adhesive material. Additional potting material is then inserted into the interior of the case through the opening in the cover to fill it, after which the opening is sealed. This process is expensive both in terms of potting material cost as well as labor costs since it is time consuming to properly pot the component.
The encapsulation by the potting material may degrade the quality of the circuit component since the potting material generally has a dielectric constant significantly greater than air, e.g. as high as four or five, and thus increases the leakage capacitance effects in the component. The potting material is not impervious to moisture and therefore if humidity proofing is required the circuit component must first be treated with a conformal coating, such as a polyurethene type available under the tradename Conap, to assure its water-tight integrity. The potting material itself serves as an adhesive to maintain the case shell cover together. However, the soft pot material is not sufficiently adhesive so that a further material, such as an epoxy, is required to seal the case shell and cover. While the potting material surrounds the elements of the circuit component to maintain them free from sudden movement through a dampening effect, this encapsulation becomes a disadvantage when it is desired to depot the component to correct a deficiency or for some other reason.
Enclosure of circuit components without potting was of course used prior to the development of potting techniques, and still continues in various forms. In one such device a steel metal case is provided. The pin leads of the circuit component are pushed through holes punched in another board to which the metal case cover is soldered. A copper clad layer of the printed circuit board is then seam soldered to the metal box to secure it. Such a construction is both expensive to fabricate and difficult to alter without damaging the component.
Another device utilizes a specific printed circuit board configuration in which the lead pins are positioned on the side of the board. The pins are placed in a box and act as clips to hold the component in position within the box. This device requires special pins which are on different centers from that of the modules meeting industry-wide standards and thus its use is specific and limited.